Miloje Makivić

Phenomenology of two channel Kondo alloys and compounds

Abstract We report realistic calculations of magnetic response and transport coefficients for a model U 4+ ion in a metal which undergoes the quadrupolar Kondo effect. We find a small critical regime for the √ T term in the resistivity ρ ( T ). The magnitude and sign of the √ T term are argued to be nonuniversal for finite crystal field splitting. Related √ T terms are found in the van Vleck susceptibility which may be observed in UBe 13 . The thermopower S ( T ) and ρ ( T ) show characteristic crystal field related peaks. S ( T ) undergoes a low-temperature sign change when the dominant fluctuation weight is to the excited f n +1 configuration from a stable f n configuration; this may help explain data for UBe 13 and CeCu 2 Si 2 . Considering the two-channel lattice in a particular infinite dimension limit, we argue that ρ ( T = 0) ≠ 0 in the absence of symmetry breaking. This may explain the reproducibly large residual resistivity of UBe 13 .

Knight shift anomalies in heavy electron materials

We calculate nonlinear Knight shift {ital K} vs susceptibility {chi} anomalies for Ce ions possessing local moments in metals. The ions are modeled with the Anderson Hamiltonian and studied within the noncrossing approximation. The {ital K} vs {chi} nonlinearity diminishes with decreasing Kondo temperature {ital T}{sub 0} and nuclear-spin--local-moment separation. Treating the Ce ions as an incoherent array in CeSn{sub 3}, we find excellent agreement with the observed Sn {ital K}({ital T}) data.

Neutron scattering study of the quadrupolar Kondo alloy Y0.8U0.2Pd3

Abstract Inelastic neutron scattering measurements have been performed on a polycrystalline specimen of Y0.8U0.2Pd3 and on the nonmagnetic analog compound Y0.8Th0.2Pd3 using unpolarized neutrons. The magnetic contribution to the inelastic spectrum at T=10 K exhibits two distinct peaks centered at energies E≃5 meV and 16 meV, attributed to broadened crystalline electric field (CEF) levels, with half widths at half maximum of ∼3 meV and ∼8 meV, respectively. The integrated intensities are consistent with Γ5 and Γ4 first and second excited state triplets lying above a Γ3 doublet ground state for tetravalent U ions in a cubic CEF. The absence of a pronounced low-energy (⩽4 meV≃kBTK) quasielastic response provides further evidence that the nonmagnetic γ3 doublet lies lowest in energy. These observations support the interpretation of the thermal, magnetic and transport properties in terms of the two-channel quadrupolar Kondo effect.

Bosons in a random potential: Evidence for new low energy excitations☆

Abstract We study the T = 0 critical properties of the superfluid-insulator transition in 2D hard core bose systems with disorder. Using quantum Monte Carlo simulations and finite size scaling on up to 64×64 size lattices we find the dynamical exponent z = 0.5 ± 0.05 and the compressibilityκ is finite at the transition. These conclusion differ from the existing scaling theory that argues for z = 2 based on a finite κ at the transition. We also find that in the presence of disorder, the boson model and the quantum rotor model are in different universality classes. Our results are suggestive of new low lying collective excitations in the disordered system that are modified from usual phonons. Such a conjecture is further supported by the anomalous temperature dependence of the specific heat deep within the superfluid phase which deviates from the expected C ν ∼ T 2 behavior for phonons.

Numerical Experiments on Frustrated Quantum Spin Systems

Thermodynamic properties of the antiferromagnetic Heisenberg model with frustration on the square lattice are investigated using quantum simulation based on the decoupled cell Monte Carlo method. The effect of frustration is introduced through an additional next-nearest neighbor antiferromagnetic interaction J 2. With increasing J 2, certain spin ordering was observed at low temperatures.